1. Field of the Invention
The invention is directed to compositions and methods for improving the performance of electrophoretic displays by modifying the electrical properties of the display cells.
2. Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. An electrophoretic fluid composed of a colored solvent with charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side or the other causing either the color of the pigment particles or the color of the solvent being seen from the viewing side.
An improved EPD technology was disclosed in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO 01/67170), U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (corresponding to WO 02/01281) and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001 (corresponding to WO02/65215), all of which are incorporated herein by reference. The improved EPD cells may be prepared by a lithographic process or by microembossing a layer of a radiation curable composition coated on a first substrate layer to form microcups of well-defined shape, size and aspect ratio. The microcups are then filled with an electrophoretic fluid and sealed with a sealing layer. A second substrate layer is laminated over the filled and sealed microcups, preferably with an adhesive layer.
For all types of electrophoretic displays, image bistability is one of the most important features. However in certain cases, the image bistability may degrade due to reverse bias. The term “reverse bias” is commonly used to describe a voltage induced by the capacitor discharge effect from a dielectric material used in an electrophoretic display. The polarity of the reverse bias is opposite of that of the applied driving voltage and therefore the reverse bias may cause the particles to move in a direction opposite from the intended direction. As a result, the display would have inferior image contrast and bistability. An example of reverse bias is illustrated in FIG. 2. The voltage sensed by the dispersion when the applied voltage drops from +40V to 0V is referred to as the “reverse bias” and its polarity is negative (opposite of the original applied voltage).
The magnitude of the reverse bias is mainly determined by the relative volume (bulk) resistivity of the dielectric material when compared with that of the electrophoretic fluid, the higher the volume resistivity of the dielectric material, the higher the reverse bias.
U.S. Pat. No. 6,525,865 discloses an electrophoretic display wherein resin in which a conductive material (carbon or metallic fiber) is kneaded and contained can be used as a material of a bulkhead or a sealer. According to the document, either the bulkhead or sealer that is made conductive can be used as a common electrode to be paired with pixel electrodes. The conductivity of the bulkhead or sealer in this case is not controlled.
U.S. Pat. No. 6,657,772 discloses that the volume resistivity of an adhesive layer can be decreased by blending a conductive filler into the adhesive composition, however, it also acknowledges that there are great difficulties in adopting this approach to achieve the volume resistivity of about 1010 ohm cm required for an adhesive layer used in an electrophoretic display. The document further states that the volume resistivity of the conductive filler should not be about two orders of magnitude less than the intended volume resistivity of the final blend and it claims that an adhesive layer has a volume resistivity in the range of about 109 to about 1011 ohm cm may only be achieved by a mixture of an adhesive material having a volume resistivity of at least about 5×1011 ohm cm and a filler having a volume resistivity not less than about 107 ohm cm.
Most conductive fillers are not transparent and require tedious grinding or milling to be uniformly dispersed into the display cell structure or other dielectric layers. Aggregation of the filler particles may results in undesirable effects such as poor image uniformity, mottling or sometimes short circuit of the display. In the case of microcup-based EPDs, incorporation of conductive filler particles into the microcup structure or the top-sealing and/or adhesive layer tends to cause problems in the manufacture of the microcups. Defective microcups may be resulted from insufficient or non-uniform degree of photoexposure during a microcup forming process (e.g., microembossing or photolithographic exposure). Moreover, if the particle size of the filler particles is relatively large as compared with the degree of surface roughness or thickness of the layer comprising the particles, damage on the embossing shim or the conductor film such as ITO/PET during embossing may be observed, particularly when the hardness of the conductive filler is higher than that of the shim material or conductor film used.